Uncooled amorphous YBaCuO thin film infrared detector

ABSTRACT

A thermal detector includes a transducer layer of semiconducting yttrium barium copper oxide which is sensitive at room temperature to radiation and provides detection of infrared radiation. In a gate-insulated transistor embodiment, a layer of ferroelectric semiconducting yttrium barium copper oxide forms a gate insulator layer and increases capacitance of the transistor or latches the transistor according to the polarization direction of the ferroelectric layer. The transducer layer may be formed as an amorphous semiconductor and deposited at room temperature by simple sputtering. The sensitive element can be incorporated into a thermal isolation structure as part of an integrated circuit.

This application is a division of application Ser. No. 08/667,628, filedJun. 21, 1996, which is a continuation-in-part of application Ser. No.08/382,200, filed Feb. 1, 1995, now U.S. Pat. No. 5,572,060, whichclaims the benefit of provisional application Ser. No. 60/000,424, filedJun. 22, 1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to infrared detectors and morespecifically to thin film amorphous infrared detectors that operate atroom temperature.

2. Description of the Background

A radiation detector is a device that produces an output signal which isa function of the amount of radiation that is incident upon an activeregion of the detector. Infrared detectors are those detectors which aresensitive to radiation in the infrared region of the electromagneticspectrum. Infrared detectors include two types of detectors, thermaldetectors and photon detectors.

Photon detectors function based upon the number of photons that areincident upon a transducer region of the detector. Photon detectors havea direct interaction between electrons and photons, are relativelysensitive and have a high response speed compared to thermal detectors.However, photon detectors operate well only at low temperatures andtherefore require refrigeration to provide sensitive detection.

Thermal detectors function based upon a change in the temperature of thetransducer region of the detector due to absorption of the radiation tobe detected. Thermal detectors provide an output signal that isproportional to the temperature of the transducer region. Sinceradiation absorption usually occurs over a wide range of wavelengths,thermal detectors are typically responsive over a wide range ofwavelengths. However, thermal detectors typically have a lowersensitivity and a slower response speed than photon detectors.

A bolometer is a thermal detector having a transducer region whoseresistance depends upon its temperature. The voltage responsivity of abolometer is a measure of the effectiveness of the bolometer atdetecting radiation. The voltage responsivity of a bolometer is definedas follows:

    R.sub.VB =dR/dT ηI.sub.b /(G(1-ω.sup.2 τ.sup.1).sup.1/2)

where I_(b) is bias current that is passed through the transducer regionof the detector, R is electrical resistance of the transducer region ofthe detector, η is absorptivity of electromagnetic radiation incidentupon a surface of the transducer region of the detector, G is thecoefficient of thermal conductance of heat away from the transducerregion of the detector, ω is angular modulation frequency ofelectromagnetic radiation incident upon the transducer region of thedetector, T is the temperature of the transducer region, and τ is thethermal time coefficient of the transducer region of the detector. τ isequal to C/G where C is the heat capacity of the transducer region ofthe detector.

Normalized voltage detectivity D^(*) _(VB) for a bolometer is anothermeasure of the sensitivity and is defined by

    D.sup.*.sub.VB =(R.sub.VP (Δf·A).sup.1/2)/V.sub.n

where Δf is the frequency bandwidth (usually of an amplifier) associatedwith the bolometer and V_(n) is the noise voltage of the output signalof the bolometer. High detectivity therefore requires (1) a low noisevoltage V_(n) and (2) a high responsivity R_(VP).

Recently, microbolometer array structures have been fabricated forthermal imaging. The new generation bolometers are made from vanadiumoxides, amorphous silicon and high T_(c) superconductors. The first ofthese is a semiconductor including V₂ O₅ and VO₂. Preparation ofvanadium oxides in both bulk monocrystalline form and thin film form isvery difficult given the narrowness of the stability range of the oxide.Both oxides show a temperature induced reversible crystallographictransformation from a semiconductor to a metal as the temperature isincreased. This phase transition can be utilized to build opticalswitches where laser pulses induce heating. However, for infraredradiation detection applications, the highly absorbing and reflectingcharacteristics of the metal phase are undesirable.

As for amorphous silicon, although it shows a reasonable TCR(Temperature Coefficient of Resistance), and excellent compatibilitywith existing CMOS technology, it has been shown to exhibit a very large1/f noise, and therefore low sensitivity.

A pyroelectric detector is a thermal detector incorporating apyroelectric material as the transducer material. Pyroelectric materialshave an electric polarization and thereby a dielectric constant whichare functions of temperature. As the temperature of the pyroelectricmaterial changes, the electric polarization of the pyroelectric materialchanges. Insulating pyroelectric materials generate a surface chargethat is proportional to their electric polarization because of thepyroelectric effect. A pyroelectric detector may be formed from acapacitor which has a pyroelectric material as its dielectric.

The responsivity of a pyroelectric detector R_(VP) is defined as theratio between the output voltage of the pyroelectric detector and theradiant power that is incident upon the pyroelectric detector. Thenormalized voltage detectivity of pyroelectric detector D^(*) _(VP) isdefined as:

    D.sup.*.sub.VP =(R.sub.VP (Δf·A).sup.1/2)/Vn

where R_(VP) is the voltage responsivity of the pyroelectric detector,V_(n) is the noise voltage of the pyroelectric detector, Δf is thefrequency bandwidth (usually of an amplifier) that is associated withthe pyroelectric detector, and A is the area of a surface of thepyroelectric material which is heated by the incident radiant power.

Another measure of the sensitivity of a pyroelectric detector is thepyroelectric figure of merit M_(r) which is defined as follows:

    M.sub.r =p/(ρc.sub.p (ε.sub.r Tan(δ)).sup.1/2)

where c_(p) is the specific heat of the heated portion of thepyroelectric detector, ρ is the density of heated portion of thepyroelectric detector, ε_(r) is the dielectric constant of thepyroelectric material (where ε=ε_(r) ×ε₀ and ε₀ is the permitivity offree space), and δ=(σ/(ω·ε))^(1/2), where σ is the conductivity of thepyroelectric material and ω is the angular frequency at which incidentradiation falling upon the pyroelectric detector is modulated. Thechange in permitivity and electric polarization of a pyroelectricmaterial or layer provide measures of the change in temperature of thepyroelectric material.

The change in electric polarization of a pyroelectric material providesa pyroelectric current which is defined as the change in surface chargeon the surface of the pyroelectric material per unit time that isgenerated by the change in magnitude of the electric polarization of thepyroelectric layer.

The latest generation pyroelectric devices are thin films deposited onSi air-gap bridge structures for low-thermal mass, low thermalconductivity and hence increased responsivity. Since these devices aremade on silicon, they can be directly integrated into silicon signalprocessing circuitry without the need for pump-bonds or wires.

Prior art thermal detectors (i.e., bolometers and pyroelectricdetectors) have been characterized by a D^(*) for infrared detection ofless than 10¹⁰. There is a continuing need for higher sensitivity, lowernoise, and therefore higher detectivity thermal detectors.

Many of the prior art infrared detector materials use a transducingmaterial for which there is no suitable thin film deposition technology.For example, barium strontium titanate transducers, which have been usedas pyroelectric transducers, have been prepared by first forming bulkceramics and then mechanically thinning the bulk ceramics in order toreduce their heat capacity.

It is desirable to have a transducer material that is compatible withthin film deposition and processing technologies. In addition, it isdesirable to have a transducer material which can be vacuum deposited asa thin layer or film (i.e., having a thickness of less than a fewmicrons) without requiring significant heating of the substrate.

Another technology which is unrelated to sensors per se, but which iscoincidentally addressed by the present invention, is gate-insulatedtransistor technology. Transistors operate by switching a semiconductorconductive channel between a conducting "open" state and anon-conducting "closed" state. Gate-insulated transistors use thevoltage applied from a gate electrode to affect the potential of aconductive channel in the semiconductor. The potential applied to theconductive channel by the gate electrode determines whether chargecarriers (i.e., electrons or holes) travel along the conductive channelwhen an electromotive force exists along the conductive channel. Thus,the voltage of the gate electrode determines whether the conductivechannel is in the conducting "open" state or the nonconducting "closed"state.

Typically, the voltage applied to the gate electrode must becontinuously maintained in order to maintain a potential at theconductive channel and the particular switching state of the transistor(i.e., either the "open" state or the "closed" state). An electricallyinsulating layer, called a gate insulator layer, separates the gateelectrode from the conductive channel of the transistor.

When the gate insulating material includes a layer of ferroelectricmaterial, the polarization of the ferroelectric layer affects thepotential of the conductive channel. Therefore, it is possible to switchthe conductive state of the channel of the transistor between the "open"state and the "closed" state, by inverting the polarization of the layerof ferroelectric material.

The polarization of the layer of ferroelectric material may be invertedby applying a voltage pulse of suitable polarity to the gate electrodeto generate an electric field at the ferroelectric layer that willinvert the polarization of the ferroelectric layer. When a voltage pulseis applied to the gate electrode which switches the ferroelectric stateof the ferroelectric gate insulator, there is a change in the electricpolarization of the ferroelectric gate. This electric polarizationchange results in a change in the potential of the conductive channel ofthe transistor.

Gate-insulated transistors including a ferroelectric insulator layer inthe gate insulator are also desirable for memory cells. A ferroelectriclayer in the gate insulator attracts or repels charge carriers in theconductive channel of the transistor thereby providing a mechanism tostore charge. The capacity of charge storage of a gate-insulatedtransistor incorporating a ferroelectric insulator layer in the gateinsulator is greater than the capacity of a gate-insulated transistorwithout a ferroelectric layer in the gate insulator.

However, the materials that are known to be ferroelectric and which canbe deposited in a layer on conventional semiconductors such as siliconmust be deposited at relatively high deposition temperatures. These highdeposition temperatures are detrimental to the structures that arenecessary to form devices in conventional semiconductor substrates, suchas silicon substrates. Thus, there is a long felt need for aferroelectric material which is compatible with conventionalsemiconductor processing technology so that it can be used as aferroelectric layer in a gate insulator for silicon based transistors.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a thermal detector with ahigh detectivity and that does not require cooling.

It is another object of the invention to provide a bolometer with a highdetectivity and that does not require cooling.

It is another object of the invention to provide a pyroelectric detectorwith a high detectivity and that does not require cooling.

It is another object of the invention to provide a gate insulatedtransistor incorporating a ferroelectric layer in the gate insulator.

It is another object of the invention to provide a bolometer using anamorphous semiconducting YBaCuO thin film.

The present invention provides bolometers and pyroelectric detectorswhich can operate at room temperature with a transducer including alayer of material which has a resistivity at twenty degrees centigradeof at least 0.1 Ω-cm, a temperature coefficient of resistance at twentydegrees centigrade of at least 0.4 percent per degree centigrade, adielectric constant of at least 50, a change in dielectric constant withchange in temperature of at least at least 0.20 per degree centigrade.

The present invention also provides a transistor having a gate insulatorwhich includes a layer of material which is ferroelectric at roomtemperature and which can be deposited at temperatures that arecompatible with conventional semiconductor processing.

The layers of materials provided by the present invention consistessentially of yttrium or any of the rare earths or lanthanum orcombinations thereof, barium or strontium or calcium or combinationsthereof, copper, and oxygen (yttrium or any of the rare earths orlanthanum or combinations thereof, barium or strontium or calcium orcombinations thereof, copper, and oxygen are hereinafter referred toherein as "YBCO") in one or more solid state phases that are crystallineor polycrystalline, such that the layers exhibit semiconductingresistance versus temperature behavior, which means that the resistivityis relatively high compared to metals and compared to crystallineconducting orthorhombic YBa₂ Cu₃ O₇, and that the resistance of thelayers increase with decreasing temperature. Since lanthanum and therare earth elements substitute for Y in YBCO phases, lanthanum or any ofthe rare earth elements may partially or completely substitute foryttrium in the layers of YBCO. These rare earth elements include Ce, Pr,Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

The resistivity at twenty degrees centigrade of the YBCO layers of theinvention is greater than 0.1 Ω-cm and more preferably greater than 1.0Ω-cm. The range of resistivity for the YBCO layers are at least 1.4 to15 Ω-cm. The temperature coefficient of resistance (temperaturecoefficient of resistance means 1/R×dR/dT, R being resistance and Tbeing temperature) of the YBCO layers of the invention at twenty degreescentigrade are greater than 0.4 percent per degree Centigrade, andpreferably greater than 2.0 percent per degree Centigrade. The range oftemperature coefficient of resistance values for the YBCO layers of theinvention that have been measured is 0.4 to 4.02 percent. Improvedmaterials processing will provide a YBCO layer with temperaturecoefficients of resistance in the range of five to ten percent at twentydegrees centigrade. The dielectric constant of the layers of materialsof the invention at 20 degrees centigrade are greater than 50 and atleast as high as 87. The change in dielectric constant with change intemperature at twenty degrees centigrade is greater than 0.2 and ispreferably greater than 0.5.

The solid state phases in the YBCO layers of the invention includeprimarily the semiconducting phases in the YBCO phase diagram,specifically including the tetragonal phase of Y₁ Ba₂ Cu₃ O_(6+x) where0≦X<0.5, crystalline phases having the compositions Y₂ BaCuO₅₊δ in which-0.5<δ<0.5, and YBa₃ Cu₂ O_(x), and amorphous semiconducting phases ofYBCO.

Moreover, the YBCO layers of the invention may include, (although thisis detrimental to the advantageous properties of the material), a smallamount of metallic crystalline or polycrystalline solid state phases ofYBCO. Metallic here means that the resistivity is less than about 0.05Ω-cm at twenty degrees centigrade and decreases with decreasingtemperature. The metallic phases in the YBCO phase diagram include Y₁Ba₂ Cu₃ O_(7-x), where 0≦X<0.5, Y₁ Ba₂ Cu₃.5 O_(7+x) where 0<x≦1, Y₁ Ba₂Cu₄ O_(8+x) where 0≦x≦1, Y₂ Ba₁ Cu₁ O₅, and Y₁ Ba₃ Cu₂ O_(x). Theresistivities of these metallic phases are all less than about 0.05ohm-cm at twenty degrees centigrade.

Since the resistivities of the conducting phases are typically muchlower than the resistivities of the semiconducting phases of YBCO, thevolume fraction of the YBCO layers of the invention that is conductingcrystalline phase must be small enough so that the conductingcrystalline phase does not form a conductive percolation path throughthe layer. This generally requires that the volume fraction of the YBCOlayers that are conducting phases be less than 20 percent and morepreferably less than 5 percent. Most preferably there is zero volumefraction of conducting phases in the YBCO layer of material. Conversely,the volume fraction of the YBCO layers that are semiconducting phaseshould be at least 80 percent, more preferably at least ninety fivepercent, and most preferably one hundred percent.

An important aspect of the present invention is that the YBCO layers maybe deposited without heating the substrate and have the resistivity,dielectric constant, temperature coefficient of resistance, and changein dielectric constant versus temperature mentioned above. The substratetypically remains below 100 degrees centigrade during deposition whenthe substrate is not heated during deposition. The YBCO layers may bedeposited by reactive coevaporation, sputtering, or pulsed laserdeposition. In addition, chemical vapor deposition may also be used toform YBCO layers. YBCO films deposited at room temperature in thismanner tend to be amorphous to polycrystalline on a fine scale.

While the temperature of the substrate may be heated during depositionin order to improve the crystallinity of the depositing layer ofmaterial, heating the substrate during deposition is not essential.

A post deposition annealing in controlled partial pressure of oxygen mayalso be used in order to control the oxygen composition of the YBCOlayers. The post deposition anneal, in a low pressure of oxygen candeplete oxygen from the YBCO layers, resulting in semiconducting YBCOlayers. However, annealing is not essential. Thus, the present inventionprovides for deposition of YBCO layers at temperatures that arecompatible with conventional semiconductor processing. The YBCO layersare useful due to their high resistance and high temperature coefficientof resistance (for bolometer applications), high dielectric constant andtemperature coefficient of dielectric constant (for pyroelectricdetector applications), and ferroelectric property (for transistorapplications).

An amorphous semiconducting thin film can also be used as the sensitivematerial for a bolometer. Typically, the semiconducting YBCO thin filmcan be fabricated into a thermal isolation structure to reduce thethermal conductivity between the substrate and the thin film sensitiveelement. The incident photon flux is chopped with a mechanical orelectro-optical chopper. The ability to depositsemi-insulating/semiconducting thin films at ambient temperature by rfmagnetron sputtering is particularly attractive. By avoiding theannealing step the amorphous structure is formed. The lack of anannealing procedure also is more compatible with CMOS technology.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1(a) is a side sectional view of a bolometer incorporating a layerof YBCO material according to the present invention;

FIG. 1(b) is a plan view of the integral detector structure in FIG.1(a);

FIG. 2(a) is a side sectional view of a pyroelectric detectorincorporating a layer of YBCO material according to the presentinvention;

FIG. 2(b) is a schematic of the equivalent circuit of the integraldetector structure in FIG. 2(a);

FIG. 3 is a side sectional view of a semiconductor transistorincorporating a layer of YBCO material in the gate insulator accordingto the present invention;

FIGS. 4(a) and 4(b) are partial side sectional views showingintermediate structures during a process for fabricating a suspendedintegral transducer structure;

FIG. 4(c) is a partial side sectional view showing a suspended integraltransducer structure;

FIG. 4(d) is a partial plan view of the suspended integral detectorstructure of FIG. 4(c);

FIGS. 5(a) to 5(d) are side sectional views of intermediate structureswhich are sequentially formed during a process of fabricating thesemiconductor transistor shown in FIG. 3;

FIG. 6 is a block diagram of electronics for measuring resistivityversus temperature of a YBCO layer in a bolometer according to thepresent invention;

FIG. 7 is a graph showing resistance versus temperature for a layer ofYBCO material according to the present invention;

FIG. 8 is a block diagram of electronics for measuring capacitanceversus temperature for a pyroelectric detector structure incorporating alayer of YBCO material according to the present invention; and

FIG. 9 is a graph showing capacitance per unit area versus temperaturefor a layer of YBCO material according to the present invention.

FIG. 10 is a graph showing the X-ray diffraction pattern of an amorphousthin film according to the present invention.

FIG. 11 is a graph showing the RAMAN scattering spectrum of an amorphousthin film according to the present invention.

FIG. 12a is a graph showing resistance versus temperature for fourdifferent YBCO samples.

FIG. 12b is a graph showing the temperature coefficient of resistance ofthe same YBCO samples in FIG. 12a.

FIG. 13 shows a thermal isolation structure by removing supportingsilicon from beneath according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, and moreparticularly to FIG. 1 thereof, FIG. 1 shows a bolometer B having asupporting layer 1 on which is a crystalline or polycrystallinesemiconducting YBCO layer 2 according to the invention. Two conductingelectrodes 3, 3 are on the YBCO layer 2. The supporting layer 1, YBCOlayer 2, and conducting electrodes 3, 3 form the integral transducerstructure 42. Preferably, the integral transducer structure 42 issupported by the connector leads 4 which also electrically connect theconducting electrodes 3, 3 to the conducting pillars 5, 5. The height ofthe conducting pillars 5, 5 is greater than the combined thickness ofthe supporting layer 1, the YBCO layer 2, and the conducting electrodes3 so that the integral transducer structure 42 is suspended above anupper surface of the substrate 6. The substrate 6 may be a semiconductorsubstrate which includes the integrated circuit 7. Alternatively, thesubstrate 6 may have the integrated circuit 7 mounted to it. Theintegrated circuit 7 is electrically connected to the YBCO layer 2 sothat it can send electrical signals to the YBCO layer 2 and receivesignals from the YBCO layer 2 via the conducting pillars 5. A shutter orchopper S is schematically indicated above the exposed surface of theYBCO layer 2 for alternately exposing the YBCO layer 2 to radiation andblocking the YBCO layer from radiation. The supporting layer 1 is aninsulator.

The integral transducer structure 42 is spaced from the substrate 6 inorder to provide increased thermal isolation to thereby provide thebolometer B with a high responsivity. Preferably, the thickness of theintegral transducer structure 42 (i.e., the thickness of the supportinglayer 1, the YBCO layer 2, and an electrode 3) is less than 10 micronsand more preferably less than 5 microns in order to decrease the thermalmass and thereby increase the thermal response to incident radiation.

FIG. 1(b) is a plan view of the upper surface of the integral transducerstructure 42 and shows the upper surface of the supporting layer 1,meander pattern formed by the YBCO layer 2, and the upper surface of theelectrodes 3, 3. However, the meander pattern is not essential for theoperation of the bolometer. The preferred pattern is a rectangular orsquare pattern.

The integrated circuit 7 is electrically coupled to the YBCO layer 2 viathe conducting pillars 5 and comprises means for measuring theresistance of the YBCO layer 2. The means for measuring resistancecomprises either a means for providing a voltage across the YBCO layerand a means for measuring the resulting current through the YBCO layer 2or a means for providing a current through the YBCO layer 2 and a meansfor measuring the resulting voltage drop across the YBCO layer 2. Themeans for measuring resistance also includes means for dividing thevoltage by the current.

It is not necessary for the integral transducer structure 42 toelectrically connect to the substrate 6 via the conducting pillars 5. Ifthe electrical connection between the integral transducer structure andthe integrated circuit 7 does not include the conducting pillars 5, thenthe pillars need not be conducting. The pillars provide structure forthermally isolating the integral transducer structure from thesubstrate. Preferably, the pillars project up from the substrate by atleast 100 nanometers.

It is not necessary for the means to measure the resistance of the YBCOlayer 2 to be in the integrated circuit 7 or even on the substrate 6.

The supporting layer may be formed of, for example, silicon nitride,strontium titanate, or yttrium stabilized zirconia.

FIG. 2 is a cross-section of a pyroelectric detector P including theintegral transducer structure 42a and the integrated circuit 7a. Theintegral transducer structure 42a includes the conducting supportinglayer 1a upon which is deposited the YBCO layer 2. The electrodes 3a, 3aare on the YBCO layer 2. The electrodes 3a, 3a each form a capacitorwith the conducting supporting layer 1a and have the YBCO layer 2 as adielectric between them. The areas of the electrodes 3a, 3a can bedifferent from one another so that the capacitance associated with eachelectrode 3a, 3a are not equal. While this embodiment shows twocapacitors, only one capacitor is necessary for a pyroelectric detector.

FIG. 2(b) shows an equivalent circuit formed by the two electrodes 3a,3a and the conducting supporting layer 1a which includes the capacitor23 and the capacitor 24.

The integrated circuit 7a includes means for measuring the pyroelectriccurrent generated by change in the electric polarization of the YBCOlayer 2 in the integral transducer structure 42a. The means formeasuring the pyroelectric current of the integral transducer structure42a may include means for providing an alternating current or voltage(i.e., an alternating current source or an alternating voltage source)across the electrodes 3a, 3a and means for measuring the resultingalternating voltage or current (i.e., a voltmeter or ammeter) across theelectrodes 3a, 3a. While two capacitors requiring the two electrodes 3a,3a are shown in this embodiment, only one capacitor and one electrodeare necessary for a pyroelectric detector, in which case the means formeasuring pyroelectric current only provides an alternating current orvoltage across one electrode 3a and measures voltage or current acrossthe one electrode 3a. It is not necessary to apply a bias voltage tomeasure the pyroelectric current.

Preferably the integral transducer structure 42a is less than 10 micronsthick and more preferably is less than 5 microns thick in order toprovide low thermal mass. Most preferably, the integral transducerstructure 42 is less than one micron thick.

A variation to the integral transducer structure 42a has one connectorlead connected to the conducting supporting layer 1a instead of to afirst one of the electrodes 3a. In this variation the first one of theelectrodes 3a would not be necessary. Preferably, the change in thedielectric constant of the YBCO layer 2a with change in the temperatureat twenty degrees centigrade is at least 0.2.

FIG. 3 is a cross-section showing a transistor 32 incorporating a YBCOlayer 36 as the gate insulator and silicon dioxide as the fieldinsulator layer 34, both of which are on the n type semiconductorsubstrate 33. Source electrode 35 and drain electrode 39 contact the ptype source contact region 35a and the p type drain contact region 39aand are spaced apart from one another with the gate insulator YBCO layer36 and the gate electrode 37 therebetween. The transistor channel 38 isthe region in the substrate 33 that is beneath the gate electrode 37 andthe gate insulator YBCO layer 36. The gate insulator YBCO layer 36 isferroelectric and has an electric polarization whose direction may bealigned along a first direction by application of a first voltage pulsethat is greater than a first value to the gate electrode, and whosedirection may subsequently be reversed by application of a secondvoltage pulse of greater than a second value, but of opposite polarityto the first voltage, to the gate electrode. However, the ability toreverse the direction of polarization of the ferroelectric YBCO layer 36is not necessary for the transistor T to be useful for increasing chargestorage. When the transistor T forms the charge storage capacitor of amemory cell the stored charge is increased due to the presence of theferroelectric YBCO layer 36.

The YBCO layer and integral transducer structures of the invention arecompatible with conventional semiconductor processing techniquesincluding the use of positive and negative photoresist and lift-offtechniques. The integral transducer structures of the invention may beformed by conventional thin film processing techniques including the useof photoresist etch or lift-off processes.

A method for fabricating a suspended integral transducer structure isdiscussed with reference to FIG. 4(a) to FIG. 4(d). The integraltransducer structure shown in FIG. 4(c) may be used as a bolometeraccording to the invention. While FIG. 4(a) to FIG. 4(d) illustrateformation of an integral transducer structure for a bolometer accordingto the invention, substantially the same fabrication process may be usedto fabricate an integral transducer structure for a pyroelectricdetector according to the invention. There are also alternative methods.The suspended membrane holding YBCO may be Si₃ N₄, SiO₂, YSZ, LaAlO₃,SrTio₃ or others. MgO, YSZ, LaAlO₃ or SrTiO₃ can be used as bufferlayers.

A first silicon nitride Si₃ N₄ layer, a first yttrium stabilizedzirconia layer, a YBCO layer, and a second yttrium stabilized zirconialayer are sequentially deposited onto the silicon substrate 100 and thenpatterned into the layered structure 105 which consists of the siliconnitride layer 101, the first yttrium stabilized zirconia layer 102, theYBCO layer 103, and the second yttrium stabilized zirconia layer 104, asshown in FIG. 4(a). The patterned silicon nitride layer 101, the firstyttrium stabilized zirconia layer 102, the YBCO layer 103, and thesecond yttrium stabilized zirconia layer 104 may be etched or formedusing ion milling.

Next, the second silicon nitride layer 106 is deposited onto the uppersurface of intermediate structure shown in FIG. 4(a) and is patterned toexpose a region of the second yttrium stabilized zirconia layer 104. Theexposed regions of the second yttrium stabilized zirconia layer 104 arethen etched to complete the contact cuts 107. The exposed regions of thesecond yttrium stabilized zirconia layer may be etched by ion milling. Ametallic layer is then deposited on the upper surface and the metalliclayer is then patterned to form the metal leads 108, as shown in FIG.4(b).

After the metal leads 108 have been formed, the upper surface of thesilicon nitride layer 106 is patterned and etched to form the trenches109, which extend to the upper surface of the silicon substrate 101.Plasma etching may be used to etch the silicon nitride layer 106.

FIG. 4(d) shows the relative locations of the trenches 109 to thelayered structure 105 of the integral transducer structure. The trenchessubstantially surround the layered structure 105, but leave supportregions 111, 111 of the silicon nitride layer 106. The trenches are cutthrough the silicon nitride layer 106 to the upper surface of thesilicon substrate 101. After the trenches 109 have been cut, the layeredstructure 105 is undercut by etching away part of the silicon substrate100, starting from the exposed region of the silicon substrate 100 atthe bottom of the trenches, to form the air gap 110. The regions of thesilicon substrate beneath the support regions 111, 111 of the siliconnitride layer 106 are also etched away, which leaves the support regions111, 111 supporting the integral transducer structure 112. The supports111, 111 are part of the silicon nitride layer 106 and are in turnsupported by the pillar regions 113 of the silicon substrate 100 thathave not been etched away, and that include the original upper surfaceof the silicon substrate 100.

Only one support region is necessary. However, the embodiment of FIG.4(d) shows the two support regions 111, 111 and more may be used. Theselective etchant used to etch the silicon substrate does not etchsilicon nitride. For example, potassium hydroxide may be used to etchthe silicon substrate. The suspended integral transducer structure shownin FIG. 4(c) results after the air gap 110 is formed.

It is not necessary that the substrate on which the integral transducerstructure is formed be silicon. For example, the substrate may be aIII-V semiconductor or a polyamide.

FIGS. 5(a) to 5(d) illustrate in cross-section, various stages duringone process for fabrication of the transistor T.

FIG. 5(a) shows the n type substrate 33 with the source contact 35a, thedrain contact 39a, and a silicon dioxide field oxide layer 34a. Thefield oxide layer 34a is patterned to allow for the doping of the draincontact 39a and the source contact 35a. Next, the field oxide isregrown.

FIG. 5(b) shows the regrown field oxide layer 34 on the substrate 33.Next, the field oxide layer 34 is removed from the gate region and theYBCO layer 36a is deposited on the upper surface of the structure inorder to provide the YBCO gate insulator.

FIG. 5(c) shows the YBCO layer 36a covering the upper surface of thestructure. Next, the YBCO layer 36a that is outside the gate region isremoved, contact cuts are opened for the source electrode 35 and thedrain electrode 39, and a metallization layer is deposited and patternedinto the source electrode 35, the drain electrode 39, and the metalwiring (not shown) for connecting to the transistor T of FIG. 5(d).

EXAMPLE 1 BOLOMETER

An integral bolometer transducer structure was formed by the followingsteps.

A 1000 Å thick magnesium oxide layer was formed on an n-type (100)silicon substrate by RF sputtering.

Next, a YBCO layer was then sputtered onto the magnesium oxide layerfrom a sputtering target of Y₁ Ba₂ CU₃ O₇ using 150 watts of power for 7hours and in an atmosphere consisting of 10 millitorr of argon. The YBCOlayer was about 500 nanometers thick. The substrate was not heatedduring the deposition of the YBCO layer.

Then, positive photoresist (Microposite 400-27 photoresist) was spunonto the YBCO layer at 3000 rpm and dried for 30 seconds. Thephotoresist was soft-baked at 90° C. for 30 minutes and then exposed toultraviolet light for 6 seconds with a pattern designed to form anexposed region of YBCO before being emersed in developer (MF319developer) in order to remove the photoresist from non-electroderegions. After development, the photoresist was hard-baked at 125° C.for 30 minutes.

The exposed YBCO was then removed by a wet etch using an aluminum etchfor 11/2 minutes. The remaining photoresist was removed from the YBCOlayer and connecting leads were bonded to the YBCO layer.

The resistance versus temperature of the YBCO layer of example 1 wasmeasured using the system shown in FIG. 6. FIG. 6 shows a block diagramfor measuring the resistance versus temperature of a semiconducting YBCOlayer 8 in the cryostat 9, the DC leads 10, 10' connect the YBCO layer 8to the DC current and voltage source and monitor 11. The temperaturecontroller 12 is connected to the YBCO layer 8 via the leads 13 whichare thermally sunk to the YBCO layer 8. The temperature controller 12measures the temperature at the YBCO layer 8 via the leads 13 by atemperature sensor (not shown) and provides heat to the YBCO layer 8 bysending current via the leads 13 through a heater near the YBCO layer 8.The computer 15 is connected to the temperature controller 12 and the DCvoltage and current source and monitor 11 and can thereby control thetemperature of the YBCO layer 8 and receive the data from the DC voltageand current source and monitor 11 so that the computer 15 can displaythe resistance versus temperature of the YBCO layer 8.

The resistance of the YBCO layer of example 1 at about 20 degreescentigrade was 4×10⁶ Ω. The width of the YBCO layer was 0.1 mm, thelength of the YBCO layer was 11 mm, and the thickness of the YBCO layerwas 500 nanometers. Therefore, the resistivity of the YBCO layer ofexample 1 was 1.8 Ω-cm.

FIG. 7 shows the temperature dependence of the resistance of the sampleof example 1 versus temperature measured using two different biascurrents. Using either of the two bias currents, the measured resistancefalls rapidly from around 7×10⁶ Ω at zero degrees centigrade to around2×10⁶ Ω at fifty degrees centigrade. The change in resistance per degreecentigrade is about 0.1×10⁶ Ω per degree centigrade. The resistivity ofthe sample was between 1.46 and 1.65 ohm-cm at about 20 degreescentigrade. The noise voltage was measure as 10⁻⁵ volts per root hertzat 30 hertz. The temperature coefficient of resistance of the YBCO layerwas 3.5 percent per degree centigrade and the deduced detectivity was10⁹.

The composition of the YBCO layer of the integral structure of example 1was determined by electron probe analysis to be YBa₁.44 Cu₁.65 O₄.59using a standard of YBCO on magnesium oxide. The primary phase indicatedby X-ray diffraction was the tetragonal phase with stoichiometry of YBa₂Cu₃ O₆.5 with a peak at d=2.72 which corresponds to the (110) X-raydiffraction line for this tetragonal phase.

EXAMPLE 2 Pyroelectric Detector

An integral pyroelectric transducer structure was formed by thefollowing steps.

A 3000 Å thick niobium bottom electrode was sputter deposited upon theupper surface of an n-type (100) silicon substrate.

Next, a 5000 Å thick YBCO layer was sputtered from a sputtering targetof Y₁ Ba₂ Cu₃ O₇ using 150 watts of power for 6 hours and in anatmosphere consisting of 10 millitorr of argon onto the niobium bottomelectrode. The deposited layers were then annealed at 500 degreescentigrade for two hours in vacuum, in order to deplete the depositedlayers of oxygen.

Then, a 3000 Å thick aluminum layer was deposited onto the YBCO layer byRF sputtering for 10 minutes in an atmosphere consisting of 15 millitorrof argon.

A photoresist layer of Microposit 400-27 photoresist was spun onto thedeposited layers at 3000 rpm and dried for 30 seconds. The depositedlayers were then soft baked at 90 degrees centigrade for 30 minutes andsubsequently exposed to UV light for 6 seconds. The exposed photoresistlayer was developed in MF 319 developer for one minute to formphotoresist circles of 0.9 mm diameter on the aluminum. Then, thedeposited layers were hard-baked at 125 degrees centigrade for 30minutes. The deposited layers were then emersed in a wet aluminum etchfor 3 minutes in order to remove both the exposed aluminum and theexposed YBCO. This resulted in a cylindrical capacitor having a layer ofYBCO according to the invention between a lower niobium electrode and anupper aluminum electrode.

The capacitance versus temperature of the capacitor of example 2 wasmeasured using the system shown in FIG. 8. FIG. 8 shows a block diagramfor measuring the capacitance versus temperature of a capacitor havingthe YBCO layer 25 as its dielectric. The capacitor with the YBCO layer25 was disposed in the cryostat 26 and had current leads 27a, 27b, andvoltage leads 27c, 27d connecting it to the LCR meter 30. Leads 27a and27d were connected to the aluminum electrode and leads 27b and 27c wereconnected to the niobium electrode. The LCR meter 30 providedalternating current or voltage and measured alternating voltage orcurrent. The temperature controller 28 was connected to the YBCO sample25 via the leads 29. A sensor (not shown) was thermally sunk to the YBCOlayer 25 and provided temperature data to the temperature controller 28.The temperature controller 28 provided electrical current to a resistiveheater (not shown) that was thermally sunk to the YBCO layer 25 via theleads 29. The temperature provided by the temperature controller 28 wascontrolled by the computer 31 which received voltage and current datafrom the LCR meter 30 and computed and displayed capacitance of the YBCOlayer 25 versus temperature.

FIG. 9 shows the capacitance per area versus temperature for thecapacitor of example 2 measured using the system shown in FIG. 8. Thecapacitance changes by approximately 20 nanofarads (nf) per squarecentimeter between 0 and 30 degree centigrade. This provides a change incapacitance per square centimeter of approximately 0.67 nf per squarecentimeter per degree centigrade. The three different curves in FIG. 9correspond to measurements of capacitance versus temperature using thefrequencies of 150 Hz, 200 Hz, and 250 Hz. However, for all threefrequencies, the change in capacitance per change in temperature issubstantially the same.

The composition of the YBCO layer of the capacitor of example 2 wasdetermined by electron probe analysis to be YBa₂ Cu₃ O₆.45 using astandard of either YBCO on magnesium oxide or a standard of YBCO onlanthanum aluminate. X-ray diffraction of the YBCO layer of thecapacitor of example 2 indicates its structure to primarily betetragonal having lattice constants a and b of 3.8 Å and a latticeconstant c of 11.75 Å. The primary X-ray diffraction peaks appear atd=2.72 which corresponds to the (110) peak and at d=2.35 whichcorresponds to the (005) peak of the tetragonal YBCO phase having thecomposition of YBa₂ Cu₃ O₆.5. The measured dielectric constant of theYBCO layer at 200 hertz and about twenty degrees centigrade was 87 andthe change in dielectric constant with temperature at about twentydegrees centigrade was 0.275 per degree centigrade.

EXAMPLE 3 Bolometer

A bolometer was prepared using substantially the same methods for thefabrication of the bolometer of example 1, but with the followingdifferences.

Pulsed laser deposition was used to deposit the YBCO layer on a (100)oriented lanthanum aluminate substrate which resulted in a layer of YBCOthat was oriented with its c axis perpendicular to the substrate. TheYBCO layer was then annealed at 500 degrees centigrade in vacuum for 2hours. The metals stoichiometry of the YBCO layer was determined byelectron probe analysis to be YBa₂ Cu₃. The resistivity of the YBCOlayer was determined to be approximately 12.7 ohm-cm. The change inresistance at about 20 degrees centigrade was measured to be about 6000ohms per degree centigrade. The noise voltage was measured as 10⁻⁶ voltsper root hertz at 30 hertz. The temperature coefficient of resistancewas calculated to be 2.31 percent and the deduced detectivity was 9×10⁹.

EXAMPLE 4 Bolometer

This example relates to amorphous YBCO films. YBCO films were depositedin the manner discussed above, except that the substrate was not heatedduring the deposition and that there is no post-deposition annealing.These films display semiconducting behavior, have an amorphousmicrostructure, and exhibit high detectivities and responsivities. Theamorphous structure occurs because the annealing procedure which hadresulted in recrystallization of the polycrystalline YBCO filmsdiscussed above, was omitted. Fabrication of detectors without therequirement of annealing is desirable because it provides for increasedcompatibility with other thin film technologies, such as CMOS. Thecomposition of the YBCO amorphous thin films was determined byRutherford backscattering as being between the ratios for the elementsY:Ba:Cu:O as 1:1.29:2.91:7.05 to 1:2.10:3.44:9.82, as measured byRutherford backscattering spectroscopy. The preferred stoichiometry isY₁ and Ba.sub.(0.3 to 2.5) Cu.sub.(2.0 to 3.5) O_(x) and has anamorphous microstructure. Preferably, x is between 4.00 and 9.82.However, x cannot be very accurately determined by the Rutherfordbackscattering technique and therefore may vary over a substantiallywider range than indicated herein. Six samples are described in thefollowing table:

    __________________________________________________________________________    a  YBaCuO(2000Å)--MgO(350Å)--Si                                                                 1:0.54:2.01:4.51                                                                       Amorphous                                  b  YBaCuO(2000Å)--Si  1:0.31:2.28:4.64                                                                       Amorphous                                  c  YBaCuO(2000Å)--SiO.sub.2 (8000Å)--Si                                                         1:0.37:2.43:4.57                                                                       Amorphous                                  d  YBaCuO(2000Å)--MgO(350Å)--SiO.sub.2 (8000Å)--Si                                          1:0.48:2.37:4.79                                                                       Amorphous                                  e  YBaCuO(2000Å)--LaAlO.sub.3                                                                       1:1.80:2.40:6.5                                                                        Tetragonal                                 f  YBaCuO(2000Å)--LaAlO.sub.3                                                                       1:1.99:2.61:6.25                                                                       Tetragonal                                 __________________________________________________________________________

The amorphous films were determined to be amorphous by X-ray diffraction(FIG. 10) using the two Theta and glancing incident angle techniques.However, Raman spectroscopy (FIG. 11) showed a broad peak at 571 inversecentimeters. The temperature coefficient of resistance for the amorphousfilms was determined to be about 3.5-4% per degree centigrade at 20° C.,indicating good bolometric response. The resistivity of the amorphousfilms (FIG. 12a) was determined to be greater than about 0.1 Ω-cm at 20°C. The TCR is shown in FIG. 12b. Thus, these amorphous films can replacethe previously discussed crystalline films and the disclosed bolometricstructures.

Amorphous thin film materials have a number of advantageous technicalfeatures. It is capable of long wavelength infrared detection. It mayoperate at room temperature without temperature stabilizationrequirements. It can be deposited on micromachine air-gap bridgestructures for high thermal isolation. It can be fabricated at lowtemperatures and therefore has good compatibility with existing CMOStechnology. It utilizes a mechanical chopper for radiation modulation.

Amorphous thin film thermal detectors can involve bolometric orpyroelectric applications. The ease of fabrication and the performanceobtained can exceed even those obtained in examples 1-3 above.

In order to achieve thermal isolation of the thin film element, it isintegrated into a thermal isolation structure produced by Simicromachining technology. Thermal isolation structures can be producedby either building the thermal isolation structure on top of the Sisubstrate by using a sacrificial layer such as shown in FIG. 2 or byfloating a thin film membrane by removing the supporting Si fromunderneath, such as shown in FIG. 13. In either case the YBCO sensitiveelement can be deposited onto a supporting thin film membrane which canbe suspended from the substrate by Si micromachining technology. Thesupporting membrane is typically SiO₂ or Si₃ N₄.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by letters patent ofthe United States is:
 1. A pyroelectric detector, comprising:an integraltransducer structure for absorbing infrared radiation, including:anelectrically conducting supporting layer having a supporting layer lowersurface and a supporting layer upper surface; a semiconductingtransducing layer of amorphous oxide formed from (1) at least one memberselected from the group consisting of barium, strontium, and calcium,(2) at least one member selected from the group consisting of yttrium,lanthanum, and rare earths, (3) copper, and (4) oxygen, having atransducing layer lower surface, a transducing layer upper surface, aresistance of greater than 0.1 Ω-cm at twenty degrees centigrade, achange in dielectric constant with change in temperature of at least 0.2per degree centigrade at twenty degrees centigrade, and the transducinglayer lower surface opposing said supporting layer upper surface; and afirst electrode layer having a first electrode layer lower surface, afirst electrode layer upper surface, and the first electrode layer lowersurface opposing a first region of the supporting layer upper surfaceacross the transducing layer; and means, electrically coupled via saidfirst electrode layer, for measuring pyroelectric current generated bychanges in temperature of said transducing layer.
 2. A detectoraccording to claim 1, wherein said semiconducting transducer layer has athickness of less than a few microns.
 3. A detector according to claim1, wherein said at least one member selected from the group consistingof yttrium, lanthanum, and rare earths, includes yttrium.
 4. A detectoraccording to claim 1, wherein said at lease one member selected from thegroup consisting of barium, strontium, and calcium, includes barium. 5.A detector according to claim 1, wherein said semiconducting transducinglayer also has a temperature coefficient of resistance at twenty degreescentigrade of at least 0.4 percent per degree centigrade.
 6. A detectoraccording to claim 1, wherein said semiconducting transducing layer hasa dielectric constant of at least
 50. 7. A detector according to claim1, wherein said semiconducting transducing layer has a change indielectric constant with change in temperature of at least 0.2 perdegree centigrade.
 8. A detector according to claim 1, wherein saidresistivity of said semiconducting transducing layer is greater than 1.0Ω-cm at twenty degrees centigrade.
 9. A detector according to claim 1,wherein a range of resistivity of said semiconducting transducing layeris from 1.4 to 15 Ω-cm at twenty degrees centigrade.
 10. A detectoraccording to claim 1, wherein said integral transducer structure is lessthan 10 microns in thickness.
 11. A detector according to claim 1,wherein said integral transducer structure is less than 1 micron inthickness.
 12. A detector according to claim 1, wherein saidelectrically conducting supporting layer comprises niobium, saidsemiconducting transducing layer comprises yttrium, barium, copper, andoxygen, and said first electrode layer comprises aluminum.
 13. Apyroelectric detector comprising:an integral transducer structure forabsorbing infrared radiation, including; a semiconducting transducinglayer of amorphous oxide formed from (1) at least one member selectedfrom the group consisting of barium, strontium, and calcium, (2) atleast one member selected from the group consisting of yttrium,lanthanum, and rare earths, (3) copper, and (4) oxygen, saidsemiconducting transducing layer having a resistivity of greater than0.1 Ω-cm at twenty degrees centigrade, a change in dielectric constantwith change in temperature of at least 0.2 per degree centigrade attwenty degrees centigrade; and means for measuring a pyroelectriccurrent generated by changes in temperature of said transducing layer.14. A detector according to claim 11, wherein said semiconductingtransducing layer comprises, yttrium, barium, copper, and oxygen.
 15. Adetector according to claim 11, wherein said integral transducerstructure is less than 10 microns in thickness.
 16. A detector accordingto claim 11, wherein said integral transducer structure is less than 1micron in thickness.
 17. A pyroelectric detector comprising:asemiconducting transducer layer of amorphous oxide formed from (1) atleast one member selected from the group consisting of barium,strontium, and calcium, (2) at least one member selected from the groupconsisting of yttrium, lanthanum, and rare earths, (3) copper, and (4)oxygen, said semiconducting transducing layer having a change indielectric constant with change in temperature of at least 0.2 perdegrees centigrade at twenty degrees centigrade; and means for measuringa pyroelectric current generated by changes in temperature of saidtransducing layer.
 18. A detector according to claim 17, wherein saidtransducing layer is less than 10 micron in thickness.
 19. A detectoraccording to claim 17, wherein said transducing layer comprises yttriumand barium.
 20. A pyroelectric detector according to claim 17, whereinsaid means for measuring comprises a pair of electrodes, and the pair ofelectrodes sandwich the semiconducting transducing layer between them.